The present invention relates to communication systems and more particularly to cellular communication systems and demodulators that are employed therein.
In analog cellular communication systems, each time frame is restricted to a single channel frequency, i.e. a single mobile user. The user capacity of analog cellular systems is accordingly highly restricted by the number of available frequencies.
Substantially increased cellular system capacity has been achieved with the use of digital technology. In digital systems currently proposed for digital cellular telephony, each time frame is subdivided into 6 time slots, and time division multiple access (TDMA) principles are employed to produce multiple communication channels within each time frame.
In the case of full rate voice coding, two time slots are employed for each channel, and a total of three communication channels are accordingly provided in each time frame. In the case of half rate voice coding, a single time slot is employed for each channel, and a total of six communication channels are accordingly provided in each time frame. Normally, each digital channel is assigned to the same time slots in successive time frames.
The resulting user capacity in TDMA digital systems is thus either three times or six times the user capacity of analog systems depending on the voice coding rate that is employed.
A more recently developed digital system achieves even greater system user capacity through the use of inactive channel time, i.e. voice silence time or digital speech interpolation (DSI), to create additional communication channels within the standard six time slots in each time frame. This improved digital system extends digital system user capacity.
Generally, a DSI system uses voice activity detection to determine active and quiet speech periods. Only speech spurts are transmitted and as a result a mobile unit will have its digital channel assignment (one or more slots within the frame) fixed for speech duration. In a half rate DSI system, each mobile unit may thus be assigned to different time slots in successive transmission time frames according to the actual voice activity from the mobile unit.
In the operation of a digital base station where modulated carrier signals transmitted from mobile units are received. The received signals are downconverted to baseband and convened from analog to digital. A demodulator then extracts information bits from the received signal for subsequent conversion to a voice signal which is coupled to the public telephone system.
The modulated carrier signals include noise, interference, Doppler and multipath and other error signals. In turn, the quality of cellular communications significantly depends on the effectiveness with which the demodulating procedure generates a demodulated signal free of channel induced errors and true to the original modulated signal.
With increased system complexity arising from the application of digital technology to cellular communication systems, a need has developed for more powerful and more sophisticated demodulating procedures that can produce quality or even superior real-time demodulation.
The following previously filed patent applications assigned to the present assignee disclose subject matter related to the present application and are hereby incorporated by reference:
U.S. Pat. No. 5,313,495, entitled "Demodulator for Symbols Transmitted Over a Cellular Channel", filed by Youngky Kim on May 12, 1992.
U.S. Pat. No. 5,319,677, entitled "Diversity Combiner with MLSE for Digital Cellular Radio", filed by Youngky Kim on May 12, 1992.
A significantly improved algorithmic demodulation procedure that functions as a maximum likelihood sequential estimator is disclosed in the above referenced patent applications. That algorithm provides improved demodulation performance with reduced computation requirements relative to the prior art. However, the reduced level of computation requirements still exceeds the computation capability of any currently available digital signal processor.